Subterranean oil and gas wells are formed by drilling a well bore through one or more subterranean formations which contain hydrocarbons that are to be extracted from the well. The well bore is typically drilled into the ground by operation of a drilling rig which is placed at the ground surface. A drill string fitted with a drill bit is assembled at the drilling rig and the drill bit is rotated and cuts the well bore into a soil, rock or other material or medium beneath the ground and through the hydrocarbon formation or formations. After drilling, a well casing may be installed in the well bore and the well casing is typically perforated at the location of each formation. A production string is inserted in the well bore to facilitate flow of the hydrocarbons under pressure from the hydrocarbon formation or formations, through the perforations and the production string to the surface of the well.
During the drilling operation, drilling fluid is typically pumped from the well surface through the drill string and is ejected from the drill bit at the cutting end of the string. The ejected drilling fluid then returns to the well surface through the annulus between the drill string and the well bore and is again pumped through the drill string, forming a continuous circulation loop. At the cutting end of the drill string, the pressurized and ejected drilling fluid strikes the medium, enhancing the cutting action of the drill bit and cooling and lubricating the bit. The lubricating effect of the drilling fluid also facilitates disengagement and removal or extraction of the drill bit from the medium and removal of the drill string from the well bore upon conclusion of the drilling operation.
One of the challenges which is sometimes encountered in the drilling of a subterranean hydrocarbon well, particularly under circumstances in which the well is formed in a hard rocky medium, is that large particles or pieces of the medium backflow and form an obstruction in the drill string. The obstruction prevents circulation of the drilling fluid from the well surface through the drill string, drill bit and annulus and back to the well surface. Consequently, the lubricating action of the drilling fluid at the drill bit is lost or compromised and the drill bit becomes stuck in the medium. Recovery of the drill bit and drill string from the well bore may require expensive, laborious and time-consuming retrieval operations which may additionally result in lost income due to delays in production.
Accordingly, a differential pressure release sub which can be opened via differential pressure to resume circulation of drilling fluid between a drill string or coiled tubing and a well bore in the event that the drill string or coiled tubing is inadvertently obstructed is needed.